1. Field of the Invention
The present invention relates to micro-size light emitting diodes (LEDs) and detectors and their arrays for minidisplay, UV detector arrays, imaging sensors, and hyper-bright LEDs and lighting applications.
2. Description of the Prior Art
Currently, most conventional broad area LEDs operating in the wavelength region from 530-800 nm are based on the older GaP/GaAs technologies. However, GaP/GaAs based LEDs are not especially bright below 600 nm and their emission wavelengths are too long to excite phosphors for down-conversion to make white light. Current commercially available broad area LEDs based on a new class of semiconductor materials (i.e. III-nitrides) can operate in the spectral region from green, blue to violet. The III-nitride technology may eventually be extended into the red covering the entire visible spectrum. To date, most III-nitride based LEDs are fabricated from quantum well and heterostructures of InGaN/GaN, InxGa1-xN/Inxxe2x80x2Ga1-xxe2x80x2N (x not equal to xxe2x80x2), and GaN/AlGaN. Blue-green LEDs and laser diodes use InGaN as an active medium, by taking advantage of heterojunctions and quantum wells (QW), and the tunability of the band gap in the alloys from InN (1.9 eV, 652 nm) to GaN (3.4 eV, 365 nm).
The conventional LED sizes are typically larger than 200 xcexcm by 200 xcexcm (300 xcexcm by 300 xcexcm for III-nitride LEDs in particular). The efficiencies of these conventional broad area LEDs can be further improved. Moreover, these conventional broad area LEDs are not suited to minidisplays.
A need remains in the art for micro-size light emitting diodes (LEDs) for minidisplay, hyper-bright LED, and lighting applications. A need also remains in the art for micro-size detectors for use in detector arrays and imaging sensors with high spatial resolutions and UV sensitivities.
One object of the present invention is to provide micro-size light emitting diodes (xcexcLEDs) and xcexcLED arrays for hyper-bright LED and lighting applications, and for minidisplays. Another object is to provide micro-size detector arrays for use in high spatial resolution detector arrays and imaging sensors.
A conventional broad area LED is replaced with many micro-size LEDs (xcexcLEDs) connected in a manner that they are turned on and off simultaneously for hyper-bright LED and lighting applications. For example, an array of many of these xcexcLEDs fits into the same area taken up by a conventional broad area LED. The output power of these xcexcLEDs are enhanced over the conventional broad area LEDs. The enhanced quantum efficiency in xcexcLEDs is due to the increased light extraction efficiencies. Additionally, an enhanced quantum efficiency in xcexcLEDs is also an inherent attribute due to micro-size effects as well as a more efficient usage of injected current. Furthermore, strain induced by lattice mismatch between the well and barrier materials (e.g., InGaN and GaN) is partially relieved as the lateral size of the LEDs decreases, which tends to increase the radiative recombination efficiencies in xcexcLEDs. Therefore, replacing a conventional broad-area LED with a micro-size LED array can enhance the total light output for a fixed luminous area.
Many micro-size holes (regular or irregular shapes) drilled into a conventional broad area LED (termed as inverted xcexcLEDs hereafter) are also employed to increase the extraction efficiencies of the LEDs.
Many xcexcLEDs are connected in a manner that they are addressed individually for minidisplay applications. When these micro-size arrays are reverse biased, they form detector arrays as imaging sensors with high spatial resolutions. Hereafter, the reverse biased micro-sized structures (which are xcexcLEDs when forward biased) are termed xcexcdetectors.
The present invention for improving the brightness of LEDs is applicable to semiconductor LEDs, polymer LEDs, as well as others such as organic LEDs. III-nitrides will be used as a specific example throughout the text. For devices based on III-nitrides, III-nitride quantum wells (QWs) or heterostructures are employed as an active media. Such active media include QW and heterostructures of InGaN/GaN, InxGa1-xN/Inxxe2x80x2Ga1-xxe2x80x2N (x not equal to xxe2x80x2), GaN/AlGaN, AlxGa1-xN/Alxxe2x80x2Ga1-xxe2x80x2N, and lattice matched QWs and heterostructures such as InxAlyGa1-x-yN/Inxxe2x80x2Alyxe2x80x2Ga1-xxe2x80x2-yxe2x80x2N (x not equal to xxe2x80x2, y not equal to yxe2x80x2) and GaN/InxAlyGa1-x-yN. Enhanced efficiency in the UV region will be very important for the realization of commercially viable white LEDs that currently utilize UV light excitation of phosphor materials for down-conversion to make white light. In particular, ultraviolet (UV) LEDs based on III-nitrides is currently used to generate white light by coating the chips with phosphors. Phosphors down-convert part of the shorter wavelength UV light to a longer wavelength visible light. Through color mixing, the eye sees white when two colors are properly balanced. In such an application area, the generation of highly efficient UV photons based on the present invention is particularly beneficial.
Minidisplays and imaging sensors based on III-nitrides according to the present invention could be especially useful for full color minidisplays, UV/solar blind detection, medical imaging, etc.